1. Field of the Invention
The present invention relates to a heat-sensitive recording material using a combination of a diazonium salt compound and a coupling component as color developing components and, particularly, to a heat-sensitive recording material which is stable with respect to light having longer wavelengths of from about 350 nm of a light source such as a fluorescent lamp, or the like, and which exhibits excellent storability before use, excellent density of color formation when heat-developed, and superior light fastness of heat-recorded images.
2. Description of the Related Art
A diazonium salt compound is a compound having a very high chemical activity, and reacts with a compound called a "coupling component", such as a phenol derivative or a compound containing an active methylene group, to easily form an azo dye. Also, diazonium salt compounds are photosensitive and are decomposed by the irradiation of light, thereby losing their chemical activity. For these reasons, diazonium salt compounds have been utilized for a long time as photo-sensitive recording materials such as diazo copies (see Fundamentals of Photographic Engineering, Non-Silver Salt Photography Edition, edited by The Japan Photographic Association, published by Corona Co., Ltd., pp. 89-117 and pp. 182-201(1982)).
Furthermore, diazonium salt compounds have recently been applied to recording materials of which image fixing is required by utilizing the property of diazonium salt compounds losing their activity by being decomposed by light. As a typical example, a so-called photo-fixing type heat-sensitive recording material has been proposed wherein, after images are formed by the heating and reacting of a recording material provided with a recording layer containing a diazonium salt compound and a coupling component according to image signals, the images are fixed by the irradiation of light (see Kohji Sato, et al., Journal of The Image Electronics Society, Vol. 11, No. 4, pp. 290-296 (1982), etc.).
However, such recording materials using a diazonium salt compound as a color forming component have the drawback that the shelf-life thereof as a recording material is short because the chemical activity of the diazonium salt compound is very high and the diazonium salt compound gradually heat-decomposes even in the dark, thereby losing its reactivity.
As means for improving stability of a diazonium salt compound, various methods have been proposed, and as one of the most effective means, there is a method of encapsulating the diazonium salt compound in microcapsules. By microencapsulating the diazonium salt compound, the diazonium salt compound is isolated from materials which accelerate the decomposition thereof, such as water and bases. Therefore, the decomposition of the diazonium salt compound is markedly suppressed, and the shelf-life of a recording material using the microcapsules is greatly improved (see Toshimasa Usami, et al., Journal of The Society of Electrophotography of Japan, Vol. 26, No. 2, pp. 115-125 (1987)).
A general method of encapsulating a diazonium salt compound in microcapsules is carried out as follows. The diazonium salt compound is dissolved in a hydrophobic solvent to obtain a solution (an oil phase), the solution is added into an aqueous solution of a water-soluble polymer (an aqueous phase). The mixture is emulsified by a homogenizer or the like. Due to the addition of a monomer or prepolymer serving as a wall material of the microcapsules into either one of or both of the oil phase and the aqueous phase, a polymerization reaction is conducted at the interface between the oil phase and the aqueous phase. Alternatively, a method of forming microcapsules, in which walls of a polymer compound are formed by depositing a polymer, can be carried out.
These methods are described in detail, for example, in Asato Kondo, "Microcapsules", published by Nikkan Kogyo Shinbun-sha, 1970, and Tamotsu Kondo, et al., "Microcapsules", published by Sankyo Shuppan, 1977.
For the walls of the microcapsules, various materials can be used such as crosslinked gelatin, algintates, celluloses, urea resins, urethane resins, melamine resins, nylon resins and the like.
In a case in which microcapsules have walls formed of a material such as a urea resin or a urethane resin having a glass transition temperature and the glass transition temperature is slightly higher than room temperature, the walls of microcapsules exhibit material impermeability at room temperature, but exhibit a material permeability at the glass transition temperature or higher. Such microcapsules are called "thermally responsive microcapsules" and are useful in heat-sensitive recording materials.
Namely, with a heat-sensitive recording material in which a heat-sensitive recording layer, which contains (a) thermally responsive microcapsules containing a diazonium salt compound, (b) a coupling component and (c) a base, is provided on a support, the diazonium salt compound can be kept stable for a long period of time and also color images can be easily formed by heating. Moreover, the images can also be fixed by light irradiation.
As mentioned above, by encapsulating a diazonium salt compound, the stability of the heat-sensitive recording material can be improved significantly.
However, when the diazonium salt compound itself is chemically unstable, even if the diazonium salt compound is encapsulated, there is a limit to the improvement in stability of the heat-sensitive recording material. To improve the stability of a heat-sensitive material, it is also important to improve the stability of the diazonium salt compound itself. In a conventional heat-sensitive recording material using a diazonium salt compound, after thermally printing, so-called fixing is carried out in which the recording material thus printed is irradiated with light having the absorption wavelength of the diazonium salt compound, whereby the diazonium salt compound is photodecomposed to lose its reactivity with the coupling component. Accordingly, when the heat-sensitive recording material is left for a long period of time in a light place, the photodecomposition of the diazonium salt compound proceeds. This results in the problem that the density of color formation after storing is liable to deteriorate and the like. In particular, in the case of preparing a multicolor heat-sensitive recording material formed by laminating plural heat-sensitive recording layers each containing a diazonium salt compound having a different developed color hue, there is a problem that when the diazonium salt compound in the upper layer is photo-fixed, the uncolored diazonium salt compounds existing in the lower layers are photodecomposed.